This application relates to a refrigerant system, in which a suction modulation valve (or other type of a valve which has a small controlled opening in the closed position) is provided with pulse width modulation control to adjust refrigerant system capacity. A minimum opening size of the suction modulation valve is maintained to ensure that suction pressure inside a shell of the compressor located downstream of the suction modulation valve does not decrease below a specified value. However, this minimum opening size is adjusted in response to system operating conditions to ensure that the suction pressure within the compressor is close to the allowable minimum, and is not undesirably higher.
Refrigerant systems are known, and are utilized to condition a secondary fluid. As an example, an air conditioning system cools and dehumidifies air being delivered into a climate controlled environment. Refrigerant systems generally include a compressor compressing refrigerant and delivering that refrigerant through a discharge line to a first heat exchanger. From the first heat exchanger, refrigerant passes through an expansion device and then through a second heat exchanger. The refrigerant is then returned to the compressor.
Under various conditions, a refrigerant system may provide excess of capacity to cool or heat a secondary fluid supplied to a climate controlled environment. A number of methods are known for reducing the capacity of the refrigerant system.
One known method of reducing capacity is to provide a pulse width modulation control for a suction valve located upstream of the compressor to control the amount of refrigerant moving from the second heat exchanger to the compressor. In pulse width modulation control for a suction valve, the valve is rapidly cycled (opened and closed) to limit the amount of refrigerant flowing to the compressor. This in turn limits the refrigerant amount compressed in the compressor and refrigerant flow circulating throughout the refrigerant system, resulting in a capacity reduction for the refrigerant system, and providing more efficient operation.
One challenge with regard to such operation is that the pressure within the compressor shell must not be reduced below a specified limit defined by compressor reliability considerations. As a rough guideline, it is desirable to maintain a pressure within the compressor shell of at least 1 psia. However, when the suction modulation valve is completely closed during pulse width modulation control cycle, sometimes, the pressure within the compressor shell can decrease below this specified minimum pressure. Under such circumstances, sparking can occur at the terminals for the compressor motor, which can lead to terminal damage. This phenomenon is known as a “corona discharge” effect, and is undesirable.
Thus, it is known in the prior art to provide a minimum “leakage” opening for the suction valve, while it would be otherwise closed during pulse width modulation cycle, to prevent compressor suction from entering a deep vacuum region. Also, in another approach, a branch bypass line, containing a small internal diameter capillary tube or a small orifice, around the pulse width modulation valve has been proposed in the past to prevent compressor suction from going into deep vacuum by providing an alternate small “leakage” path for refrigerant flowing into the compressor. While the prior art does provide good control of capacity, the “leakage” opening is typically sized to ensure that the suction pressure in the compression shell exceeds the specified minimum pressure at all operating conditions.
However, the downstream pressure inside the compressor shell, when the suction valve is in the closed position, changes substantially for a constant size opening, depending on the pressure upstream of the opening. The evaporator pressure can vary by at least an order of magnitude, depending on the operating conditions of the refrigerant system. Therefore, under high pressure operating conditions at the evaporator, in the prior art, the suction pressure inside the compressor would also be much higher then what can be considered desirable for the minimum pressure in order to avoid the “corona discharge” effect. Having the suction pressure well above this threshold is undesirable, since it decreases the efficiency of the refrigerant system operating in a pulse width modulated mode. Thus, the prior art could not effectively control the suction pressure inside the compressor to be just above the acceptable threshold for all operating conditions, while at the same time avoiding the “corona discharge”.